Digital oscilloscopes acquire electrical signals by quantizing samples taken at spaced time intervals and then storing such quantized samples in a memory for subsequent display as reconstructed waveforms. Until recently, digital oscilloscopes have been severely limited in apparent bandwidth because all of the samples comprising a waveform has to be taken sequentially at the sample clock rate in a single cycle of the signal due to the asynchronous relationship of the trigger event and the sample clock. This mode of operation is known in the digital oscilloscope art as single-shot acquisition.
The apparent bandwidth of signal acquisition has been extended significantly in an acquisition mode similar to equivalent-time random sampling--that is, sampling points on respective cycles of a recurring signal and reconstructing therefrom a single equivalent-time cycle of signal even though the waveform samples may have been acquired many cycles apart. A problem associated with such equivalent-time waveform reconstruction is that it takes a comparatively long time to acquire all of the relevent samples which represent the respective data points.
Another problem is that the triggering point, which is the same on each successive cycle of the signal, and the sample clock, which operates at a predetermined fixed rate, are not correlated, resulting in horizontal jitter of the displayed data points with respect to each other. This problem was addressed by U.S. Pat. No. 4,251,754 to Luis J. Navarro and Thomas P. Dagastino, which teaches jitter correction due to sample uncertainty by measuring the time interval between a trigger recognition event (produced when the signal potential passes through a selectable threshold level) and the next succeeding sample clock pulse, and then utilizing the measured value to generate an offset current in the display horizontal system which causes a horizontal shifting of each frame of the dispay thereby to place each displayed sample at its correct time position. However, this solution to the jitter problem does not lend itself well to intermediate waveform processing by a computer or the like because the correction takes place in the display system.
Another aspect of equivalent-time waveform reconstruction is the effect of the lower Nyquist limit on the sampling rate for periodic sampling. That is, if the sampling rate is less than twice the frequency of the input signal, information will be lost due to under sampling, eventually resulting in a distorted displayed waveform. Thus, it would be desirable to acquire multiple samples for each trigger signal recognition, and to provide the correct equivalent-time location for each sampled data point in the acquisition process.